The present invention relates to pumps and more particularly to centrifugal blood pumps in which impeller blades are positioned so that hemolysis is minimized.
Delicate surgical procedures require that the site of surgery remain motionless. This requirement made early heart surgery difficult to impossible as interruption of the heart's pumping action for the required length of surgical time would invariably be fatal.
During the 1960s, prolonged and non-fatal stoppage of the heart became possible by use of newly developed "heart-lung" machines. These machines consisted of a mechanical blood pump combined with a blood oxygenator. The heart-lung machines were capable of taking over the function of the natural heart and lungs for periods of up to several hours, enabling the development of techniques leading to today's extensive practice of open-heart surgery.
The first practical mechanical blood pumps used were peristaltic or "roller" pumps. The pumping action of a roller pump derives from the compression of a section of the flexible plastic tubing which carries the blood through the heart-lung machine. A moving roller presses the tubing against a semicircular platen, moving the blood forward in the tubing. The speed of the moving roller and the diameter of the tubing control the rate of blood flow.
Although the roller pump was and is simple and reliable, it has two characteristics which can endanger the patient undergoing surgery. First, if flow is inadvertently obstructed, the resulting increase in pressure produced by a roller pump may exceed the bursting strength of the tubing circuit. Second, if air is accidentally introduced into the tubing circuit, it will be pumped to the patient along with the blood. Either of these conditions may result in serious or fatal consequences to the patient.
In 1976, centrifugal blood pumps began to replace the roller pump as the "heart" of the heart-lung machine. The pumping action of a centrifugal blood pump derives from the rotation of an impeller within a pumping chamber. One impeller design associated with centrifugal blood pumps is a disk-shaped device with multiple blades positioned on a surface. The impeller is rotated about a central axis of rotation by way of a rotational drive source. After the blood enters the pumping chamber via an inlet, it makes contact with the impeller blades and is rotated along with the impeller. The impeller rotates at a predetermined speed so that a required pressure and flow rate is maintained.
Pump pressure is controlled by the rotational speed of the impeller. At operational speeds, excessive pressure cannot be produced. Additionally, the centrifugal forces in the pump form a natural air trap and, with massive introduction of air, deprime and discontinue pumping altogether. The above-mentioned safety features, and the decreased blood damage, or hemolysis, caused by centrifugal blood pumps is now widely recognized and has led to their extensive use for open heart surgery.
In the early 1980s it was demonstrated that a mechanical blood pump could be used as a heart-assist pump for patients who could not be separated from the heart-lung machine following surgery. The readily available centrifugal blood pumps were quickly adapted to this situation as well as to the more routine use during heart surgery.
The fragility of blood, however, presents several problems for the design of mechanical blood pumps. Excessive shear forces cause rupture of the red blood cells. Hemolysis is a measure of the rate at which red blood cells are damaged. Despite the risk associated with excessive forces which may cause hemolysis, constant motion and high flow velocity rates are needed (especially over local areas of friction, such as seals) to maintain required pump pressure and to prevent points of high temperature which may cause blood damage and the accumulation of clot deposits. Thus, a balance must be established between adequate rotational speed of the pump impeller and an acceptable level of hemolysis.
Previous centrifugal blood pumps have reduced hemolysis by decreasing the rotational speed of the impeller. The required pump pressure and flow rate of blood in these pumps is maintained by increasing the diameter of the pump. In other words, increasing the diameter counteracts the decrease in rotational speed of the impeller. Some pumps, however, have dimensional constraints which do not allow an increase in pump diameter. A means to reduce hemolysis without increasing the diameter of the centrifugal blood pump is not found in the prior art.